This invention relates to a sensor for producing a differential pressure signal which varies as a function of the volumetric flow rate of air through a duct system.
The sensor may, for example, be installed in the air conditioning duct system of a building and may be associated with a controller for regulating the volumetric flow of air through the duct system. The differential pressure output signal of the sensor is representative of the actual volumetric flow rate through the duct system and is transmitted to the controller for comparison with a signal representative of the desired volumetric flow. When the actual flow does not correspond to the desired flow, the controller automatically adjusts a damper or the like in the duct in order to establish the actual flow at the desired rate. The accuracy of the controller is dependent to a large degree upon the accuracy with which the sensor transmits a pressure differential signal that is representative of the actual flow rate.
In many building duct systems, the differential pressure sensor is formed in part by a conventional orifice located in the duct. High and low pressure taps are located upstream and downstream, respectively, of the orifice and transmit a differential pressure signal as air flows past the orifice and undergoes a pressure drop. In order for the differential pressure signal produced across the orifice to accurately represent the actual volumetric flow, it is necessary that a substantially symmetrical flow pattern exist at the orifice. Frequently, the orifice is located in a downstream duct section which extends at an angle to a connecting upstream duct section such that air curves into the downstream duct section and flows in an asymmetrical pattern through the entrance thereof. Under such circumstances, it is necessary either to space the orifice a considerable distance (e.g., four duct diameters) from the entrance of the duct or to use an air straightening core or the like upstream of the orifice in order to establish a symmetrical flow pattern through the orifice. In some duct systems, the downstream duct section simply is too short to permit the orifice to be spaced the required distance from the duct entrance. It thus becomes necessary to use an air straightening device in conjunction with the orifice and such use increases the material and labor costs of the overall installation.